1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving circuit for a liquid crystal panel with a memory effect having bistable states, and particularly to a liquid crystal display device in which a memory effect presented by bistable states of a liquid crystal having a memory effect is utilized to enable operation at a low voltage and reduce the power consumption.
2. Description of the Related Art
As a display device of a personal digital assistant in which the display screen is not often switched, such as used in an electronic book or electronic newspaper which has recently received much attention, a liquid crystal panel with a memory effect using a liquid crystal having a memory effect has drawn attention. Having a memory effect means that a display image on the panel can be maintained even during no power consumption.
Using the characteristics enables reduction in the power consumption of the liquid crystal display device. A ferroelectric liquid crystal, a cholesteric liquid crystal, and so on are known as materials of the liquid crystal for use in the liquid crystal panel with a memory effect.
Such a liquid crystal panel with a memory effect has a liquid crystal having a memory effect having at least bistable sates between a pair of substrates (glass substrates) which have scanning electrodes and signal electrodes on their opposed surfaces, respectively.
FIG. 5 is a plane view of enlarged portion of the scanning electrodes and signal electrodes as seen from a direction perpendicular to the substrate surface of the liquid crystal panel, in which TP1 to TP4 are scanning electrodes and SG1 to SG4 are signal electrodes. A liquid crystal having a memory effect exists between the scanning electrodes and the signal electrodes, and portions where the scanning electrodes TP1 to TP4 are opposed to the signal electrodes SG1 to SG4 with the liquid crystal having a memory effect intervening therebetween (portions where the scanning electrodes TP1 to TP4 overlap the signal electrodes SG1 to SG4 in FIG. 5) form pixels Pix (x, y), respectively.
FIG. 6 is showing the relation between the voltage applied to a liquid crystal panel and the transmittance thereof, and bistable states of the ferroelectric liquid crystal, when a ferroelectric liquid crystal and a pair of polarizing film (not shown) are arranged.
The ferroelectric liquid crystal has bistable states, which are switched by applying a positive or negative voltage exceeding a threshold value +Vt or −Vt, so that the first ferroelectric state (ON state) or the second ferroelectric state (OFF state) can be selected depending on the polarity of the applied voltage.
More specifically, during application of no voltage, the ferroelectric liquid crystal exits bistable states in the first or the second ferroelectric state, however for example, when the applied voltage exceeds the threshold value +Vt on the positive side while the ferroelectric liquid crystal is stable in the second ferroelectric state (the black image state with a low transmittance), the ferroelectric liquid crystal is brought into the first ferroelectric state (the white image state with a high transmittance). Even if the applied voltage is gradually decreased to no voltage (0V) from that state, the first ferroelectric state is maintained.
However, when the applied voltage exceeds the threshold value −Vt on the negative side while the liquid crystal is stable in the first ferroelectric state (the white image state with a high transmittance), the liquid crystal molecule is brought into the second ferroelectric state (the black image state with a low transmittance). Even if the applied voltage is gradually increased to no voltage (0V) from that state, the second ferroelectric state is maintained.
As is clear from the chart, the liquid crystal panel using the ferroelectric liquid crystal can maintain the transmittance, that is, the display state even during application of no voltage, that is, no power consumption. The characteristics mean having a memory effect.
Incidentally, the liquid crystal panel in which the pixels Pix (x, y) are formed in a matrix form as shown in FIG. 5 typically performs rewrite a screen for the multiplex driving method.
More specifically, a scanning voltage is outputted from a scanning electrode driving circuit (not shown) sequentially to the scanning electrodes TP1 to TP4 line by line, for example, to TP1, TP2, and so on, in synchronization with which, a signal voltage is outputted from a signal electrode driving circuit (not shown) to signal electrodes SG1 to SG4 in a parallel manner. Note that the signal voltage becomes in a voltage waveform corresponding to image data to be displayed at each of the pixels Pix (x, y).
Further, a pair of polarizing film (not shown) are arranged outside the liquid crystal panel such that their absorption axes are in a crossed-Nicols state so as to create the white image in the above-described ON state and the black image in the OFF state.
Next, a conventional driving method for bringing the pixels in such a ferroelectric liquid crystal panel into the white image or the black image will be described using FIG. 14. FIG. 14 shows a driving voltage waveform and a transmittance curve of a typical ferroelectric liquid crystal panel when a pixel Pix (1, 1) at the first row and first column in FIG. 5 is brought into the white image ON (W) and the black image OFF (B). In the FIG. 14, TP1 is a voltage waveform applied to the scanning electrode TP1, and SG1 is a voltage waveform applied to the signal electrode SG1.
To bring the pixel Pix (1, 1) at the first row and first column shown in FIG. 5 into the white image, during a period for displaying one screen, a reset period RS is set at the first portion, and a selection period SE for determining the display state and a non-selection period NSE for maintaining the display state are set thereafter.
First, during the reset period RS, a driving circuit outputs bipolar pulses of voltage values +VRT and −VRT as the scanning voltage to the scanning electrode TP1, and bipolar pulses of voltage values +VRS and −VRS as the signal voltage to the signal electrodes SG1.
Thereby, during the reset period RS a voltage in a composite voltage made by adding the scanning voltage to the signal voltage, so that reset pulses of the voltage values (VRT+VRS) and −(VRT+VRS) as the composite voltage waveform TS (1, 1) are applied to the pixel Pix (1, 1).
As for the transmittance, as shown at TV (1, 1), the pixel Pix (1, 1) is brought into the first ferroelectric state, that is, the white image with a high transmittance during the first half of the reset period RS because the reset pulse becomes the positive voltage exceeding the threshold value +Vt on the positive side described with FIG. 6, whereas the pixel Pix (1, 1) is brought into the second ferroelectric state, that is, the black image with a low transmittance during the second half of the reset period RS because the reset pulse becomes the negative voltage exceeding the threshold value −Vt on the negative side.
Subsequently, during the selection period SE, the driving circuit outputs a voltage value 0V and bipolar pulses at −VS and +VS as the scanning voltage to the scanning electrode TP1, and outputs a voltage value 0V and bipolar pulses at +VD and −VD as the signal voltage to the signal electrode SG1.
Thereby, the composite voltage obtained by adding the scanning voltage to the signal voltage is applied during the selection period SE and therefore becomes the selection pulses at voltage values 0V, −(VS+VD), and +(VS+VD) as shown in the composite voltage waveform TS (1, 1), and the voltage is applied to the pixel Pix (1, 1).
As for the transmittance, since the positive voltage exceeding the threshold value +Vt on the positive side described with FIG. 6 is applied as the selection pulse in the second half of the selection period SE, the second ferroelectric state is changed to the first ferroelectric state, that is, the white image with a high transmittance as shown at TV (1, 1).
Further, during the non-selection period NSE, the driving circuit outputs the voltage value 0V as the scanning voltage to the scanning electrode TP1, and the voltage value 0V or bipolar pulses at −VD and +VD as the signal voltage to the signal electrode SG1.
Thereby, the composite voltage obtained by adding the scanning voltage to the signal voltage is applied to between the scanning electrode TP1 and the signal electrode SG1 during the non-selection period NSE. Therefore the composite voltage waveform TS (1, 1) becomes the voltage value 0V or the bipolar signal pulses at −VD and +VD, and the voltage is applied to the pixel Pix (1, 1).
As for the transmittance, since the voltage of the signal pulse is smaller in absolute value than the threshold value +Vt or −Vt described with FIG. 6 during the non-selection period NSE, the first ferroelectric state determined during the selection period SE, that is the white image with a high transmittance is maintained as shown at TV (1, 1).
Note that the pulse shown by a square in the non-selection period NSE which is the voltage waveform applied to the signal electrode SG1 in FIG. 14 shows bipolar pulses at either +VD and −VD or −VD and +VD.
As described above, in the conventional driving method, the driving voltage applied between the scanning electrode and the signal electrode is composed of the reset pulses, the selection pulses and holding pulses so that any DC component does not remain during a period for displaying one screen, thereby prevent deterioration in image quality. However, the driving voltage requires nine level values (0V, ±VS, ±VD, ±VRS, and ±VRT). Further, because of bipolar pulses, the peak-peak value (±(VRT+VRS) in FIG. 14) needs to be twice the voltage to which the liquid crystal reacts.
As described above, according to the conventional driving method, many voltage values and high voltage values have been required to drive the liquid crystal panel with a memory effect, leading to complicated configurations of the scanning electrode driving circuit (row driver IC) for outputting the scanning voltage and the signal electrode driving circuit (column driver IC) for outputting the signal voltage, moreover leading to require an IC of high-voltage process, therefore the display device has been high cost.
Hence, the inventor previously invented a liquid crystal display device and a driving circuit for a liquid crystal panel with a memory effect disclosed in Patent Document 1.
disclosed in Patent Document 1.
According to that invention, the driving circuit driving the liquid crystal panel with a memory effect to cause pixels to display image data is configured to apply voltages (showing for convenience, scanning electrodes TP1 and TP2, signal electrodes SG1 and so on in FIG. 15) in driving waveforms each composed of a voltage value 0V and positive or negative unipolar voltage (positive unipolar voltage shown in FIG. 15) to all of the scanning electrodes and the signal electrodes of the liquid crystal panel with a memory effect.
Further, the image data displayed at each pixel is displayed during a plurality of scanning periods F1 and F2, and composite voltage waveforms (TP1-SG1), (TP2-SG1) as shown in FIG. 15 such the composite voltages made by adding the scanning voltage to the signal voltage are applied to the pixels TS(1, 1), TS(2, 1), then the applied voltages are made alternating in the plural scanning periods.
Thus, it is possible that the driving waveforms of the scanning voltage and the signal voltage outputted from the driving circuit to drive the liquid crystal panel with a memory effect become positive or negative unipolar, the number of the voltage levels constituting the voltage waveforms, that is, the kinds of the voltage values of both the scanning voltage and the signal voltage are three or four values, and the voltage waveforms can be made by simple circuit.
Accordingly, the scanning electrode driving circuit (row driver IC) and the signal electrode driving circuit (column driver IC) can be reduced in size and manufactured at low cost. This allows a liquid crystal display device having the liquid crystal panel with a memory effect to be provided at low cost.    Patent Document 1: JP 2006-30964A (US 2006/0012591 A1)
As described above referring FIG. 15, the liquid crystal display device and the driving circuit for the liquid crystal panel with a memory effect disclosed in Patent Document 1 are configured to display the image data displayed at each pixel of the liquid crystal panel with a memory effect during the plural scanning periods, that is, the first scanning period F1 and a subsequent scanning period F2, and the voltage applied between the scanning electrode and the signal electrode at a portion forming the pixel is made alternating in the plural scanning periods F1 and F2.
The scanning period F1 is composed of a reset period RS for bringing the liquid crystal having a memory effect at each pixel into a first stable state, a selection period SE1 for bringing it into the first stable state or a second stable state, and a holding period NSE1 for holding the stable state thereafter, and during the scanning period F2, the stable state held during the scanning period F1 is maintained as it is.
Here, in either case in which the pixel is brought into the white image or the black image, the pulse waveform of the signal voltage applied to the signal electrode during the reset period and the pulse waveform of the scanning voltage applied to the scanning electrode during the selection period are made the same waveform with the same pulse width and same pulse voltage value, whereby the polarity of the composite voltage applied between the scanning electrode and the signal electrode can be inverted during the reset period and the selection period.
Incidentally, in the composite voltage waveform (TP2-SG1) when the black image is selected, the absolute value of the positive selection pulse is smaller than the absolute value of the negative reset pulse. Accordingly, to appropriately make alternating also for this portion, the scanning period F2 is provided after the scanning period F1.
More specifically, the potential being reference (reference potential) is made different between the scanning period F1 and the scanning period F2 in the driving waveforms of both the scanning voltage and the signal voltage, so that the complete alternating driving is compensated during the two scanning periods.
If the selection period for selecting the stable state of the pixel of the liquid crystal having a memory effect according to the image data is placed in the first scanning period F1 as described above, the signal pulse at the voltage value VD composed of one of positive or negative polarity is kept applied during the subsequent scanning period F2.
When even such a small pulse is continuously applied, the memory effect of the liquid crystal layer deteriorates, leading to a problem of a change in image. Further, since the driving waveform having a reference potential varying each scanning period, the driving circuit is complicated.